Currently, the continuous casting process is widely used in the metallurgy of iron and steel. As molten metal emerges from a converter furnace or vessel, it is poured into a ladle of high capacity, then poured into a distributor or tundish of lesser capacity which permits continuous delivery of the metal into one or more bottomless molds for solidification therein.
The flow of molten steel from the tundish to the mold or molds is regulated and/or stopped by various means. At the present time, the most widely used apparatus in Europe for controlling metal flow is the stopper rod. A stopper rod is an elongated piece of refractory material that extends downwardly into the molten steel. The lower end of the stopper rod bears against the seat of a casting orifice provided in the bottom of the tundish. An arm fixed to the upper portion of the stopper rod makes it possible to selectively move the lower end of the stopper rod a regulated distance from the seat of the pouring orifice and, thus, meter or completely stop the flow of molten metal to the molds. Rotary valve devices are also known, in which the flow of metal from the tundish is regulated by selective rotation of the device.
The upper surface of the molten metal contained both in the ladle and in the tundish is usually covered with a protective covering powder, generally of a synthetic nature. This covering powder has several functions. The powder prevents oxidation of the molten metal by isolating it from the ambient air, it reduces the heat losses from the melt and it also traps inclusions that rise to the surface of the molten steel.
Nevertheless, the use of a covering powder is not without its disadvantages. Indeed, this floating powder layer causes erosion of the stopper rod at the level of its line of immersion in the steel sometimes referred to as the "slagline". This erosion is of serious concern at present since the powders currently used are quite chemically aggressive relative to the refractory materials used to produce the stopper rods. A layer of slag floats on the surface of the molten metal contained in the ladle. This slag originates from the impurities contained in the iron ore and which, being lighter than the steel, rise to its surface when it is melted. To the greatest extent possible, an attempt is made to prevent the slag contained in the ladle from passing into the tundish by closing the ladle before the slag begins to pass into the tundish. Despite this precaution, it is inevitable that a certain amount of slag passes into the tundish. This slag also has a corrosive action on the refractory stopper rod in addition to covering powder.
The combined erosion of the slag and the covering powder causes a reduction in cross-section and subsequent weakening of the stopper rod at its immersion line in the molten metal. This gradual reduction in cross-section structurally weakens the stopper rod and will eventually cause its rupture. Such a rupture leads to a loss of control of the metal flow from the distributor vessel. All the metal contained in the tundish is then lost and can cause considerable damage to the continuous casting mold, as well as the entire casting installation, resulting in considerable expense due to lost production, and clean-up costs. Such inadvertent stopper rod failure also involves considerable physical risk to the personnel in the vicinity of the tundish resulting from the uncontrolled flow of molten steel. It is, thus, absolutely necessary to avoid any possibility of stopper rod rupture.
Because of these safety concerns and attendant operational problems, it is common to interrupt the casting sequence as soon as erosion of the stopper rod presents a risk of rupture. Erosion of the stopper rod is thus a factor that limits the duration of the casting sequence; that is, the number of ladles that can be cast in the same sequence. It is readily understood that the cost per ton of steel can be reduced if the casting sequence is extended. Thus, the stopper rod life has a direct effect on steel production costs.